The Federal Aviation Administration (FAA) requires that obstructions to aircraft navigation, such as towers, cables and tall buildings be fitted with visibly perceivable elements to render these structures highly visible to approaching aircraft. FAA Advisory Circular 150/5345-43 forms a specification of technical requirements for these lights in the United States. Within Advisory Circular 150/5345-43 there exists a requirement for a medium-intensity flashing red obstruction light system, designated the “L-864” and a medium-intensity flashing white obstruction light, designated the “L-865.” These obstruction lights are to be placed in accordance with a set plan at levels on all obstructions that are potential hazards to air navigation.
For the L-864 obstruction light, at all radials throughout a 360 degree azimuth, there must be a peak effective intensity of 2,000±25 percent candela. There must also be a minimum effective intensity of 750 candela throughout a minimum vertical beam spread of 3 degrees. For the L-865 obstruction light, at all radials throughout a 360 degree azimuth, there must be a peak effective intensity of 20,000±25 percent candela during operation at day and twilight conditions, and 2,000±25 percent candela during night conditions. The L-865 obstruction light also includes a minimum vertical beam spread of 3 degrees.
While visible-light obstruction lighting performance standards are well established, there are no standards for compatibility of obstruction lights with night vision imaging systems (“NVIS”) commonly used by military and emergency/rescue air operators. NVIS systems are typically configured to receive and amplify low-level infrared radiation to form a “night vision” image, which improves air operators' ability to navigate at night, especially in poor visibility conditions. The lack of performance standards for compatibility with NVIS systems is particularly problematic for obstruction lights that utilize light emitting diodes (LEDs), since LEDs generate considerably less heat (and thus less infrared radiation) than conventional incandescent and xenon strobe light sources, making them difficult to see with NVIS. Providers of obstruction lighting systems have attempted to solve this problem by adding some infrared light emitting diodes (IRLEDs) to their visible-light obstruction lighting systems. However, since these obstruction lighting systems are optimized to emit visible light they are often poor emitters of infrared light. There remains a need for an obstruction lighting system that provides satisfactory light emissions for both visible light and infrared light.